Saturday, October 17, 2015

Charging up my solar heating system

South end of my workshop showing 4 solar collectors
It is mid October and we have had our first few frosts here in Maine and they had a few inches of snow in western Maine yesterday.  This means that it is time for me to fill the solar collectors on my workshop heating system with antifreeze.  The heating season lasts until mid-May here and in the winter temperatures rarely get above freezing for up to three months.  My workshop building is super insulated and I heat it with a combination of solar, propane and a wood stove.  I use a cord of wood that I cut and split myself from my backwoods to heat the downstairs area, keeping it around 60°F and higher if I am actively working in the space.  I use the solar/propane primarily to heat my second-floor office where I am more sedentary and set the temperature around 70°F or so during my working hours.

Pumping a bucket full of antifreeze into the collectors
To fill the collectors, it takes around 6 gallons of 50% Dow Frost antifreeze.  I use a small electric pump and hoses to pump this fluid into the system and pressurize it to 10 psi.  It takes a while to eliminate all the air bubbles from the system so I leave the pump circulating for a while to push the bubbles out of the plumbing and collectors.
DTC-D Solar Differential Temperature Controller
Once the system is filled, I connect the 3 small solar panels on the roof to the Differential Temperature Controller that I designed and manufacture (shown above).  This device monitors the temperature of the collectors and the storage tank and only activates DC circulation pumps when the collector is hotter than the storage tank by at least 10°F.  Powering the circulation pumps directly from solar panels guarantees an efficient and reliable heating system.

You can see live performance charts and gauges of my heating system on this page of my website.  On a good sunny day in the winter the system can raise the 80 gallon storage tank temperature by over 40°F.  The stored heat is then fed to baseboard radiators at night as needed.  If the stored water temperature drops below 140°F a Bosch Aquastar boiler automatically makes up the difference to ensure that heating water is always delivered to the radiators at 140°F.


  1. Have you considered filling the solar system sooner, defeating the propane heater, and running some heat into the thermal mass of the floor? How much is there? Square footage * thickness * 22.5 = BTU / °F. for example 240 ft^3 (my estimate for your slab) of concrete is 5400 BTU for every °F you raise the temp of the floor.

  2. Actually, I have not used the hydronic floor heating loop for the last few years. 1 cord of wood suffices to keep the ground floor above 50F, and I spend most of my time in the office. A little extra firewood can make the ground floor warm very quickly as needed.


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